1. Field of the Invention
The present invention relates to a three-phase power supply, and more particularly to a three-phase power supply with a three-phase three-level DC/DC converter.
2. Description of Prior Art
With the development of power semiconductor technology, many electronic products are now light-weight, thin, and compact, and this trend will continue. The traditional linear power supply has cumbersome isolated transformers and heat sinks therein, thus the linear power supply has already been eliminated gradually due to low efficiency. Instead of the linear power supply, the switching power supply is used for high-frequency operations. The switching power supply has a lot of advantages, including smaller volume, lighter weight, and higher efficiency.
In general, the switching power supply adopts a traditional hard switching technology. When the operation frequency increases, therefore, the switching losses of turning on and turning off the power switching elements will increase with frequency. However, this will result in large heat losses to reduce efficiency and lifetime of the switching power supply. In addition, the heat dissipating apparatus is needed to install, while volume and costs increase. More particularly, non-ideal effect of switching the power switching elements causes problems of higher voltage and current stress as well as electromagnetic interference.
In order to overcome the above-mentioned problems, a soft switching technology is applied to various power electronic products. The soft switching technology can be broadly classified into two types: (1) zero voltage switching (ZVS), and (2) zero current switching (ZCS). The ZVS is achieved to reduce the voltage across the power switching element to zero before turning on the power switching element, whereas the ZCS is achieved to reduce the current through the power switching element to zero before turning on the power switching element. The soft switching technology—ZVS or ZCS, is achieved by producing a zero power of multiplying the voltage across the power switching element and the current through thereof. Accordingly, this will reduce switching losses, increase efficiency, and reduce noises of the power switching elements. However, the ZCS is achieved to cause switching losses due to electric charges stored in the power switching elements. Hence, the ZVS is preferred for high-frequency operations.
Accordingly, it is desirable to provide a three-phase power supply with a three-phase three-level DC/DC converter to reduce withstanding voltage of the power switch elements, further the zero-voltage switching (ZVS) is achieved by the isolated transformer and the power switch elements through the PWM (pulse width modulation) technology to increase the efficiency of the DC/DC converter.